Opacifying polymer particles

ABSTRACT

An aqueous dispersion of pigmented opacifying polymer particles, the particles comprising: i. an inorganic core comprising dispersed inorganic pigment particle and a dispersant ii. a first layer of base-swellable polymer comprising acid groups encapsulating the core iii. a second layer of non-base-swellable polymer encapsulating the first layer wherein the dispersant comprises or consists of hydroxyl moieties; and carboxyl groups derived from itaconic acid.

REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase of PCT/EP2013/076815, filedon Dec. 17, 2013, and claims the benefit of EP Application No.12198597.2, filed on Dec. 20, 2012.

This invention relates to improved opacifying polymer particles; aprocess for making the improved polymer particles and compositions, inparticular coatings, comprising the polymer particles.

Pigments are ubiquitous components in many compositions such as plasticcomponents in the form of structural elements for example window frames;and coatings, such as architectural, automotive and marine coatings, infact wherever colour or white is required. Of the pigments, titaniumdioxide is probably the single most common pigment in commercial use.

In the field of architectural paints consumers prefer white or pastelcolours to deep colours. This is especially so on large areas such aswalls and floors. Being mainly white, such colours contain very highlevels of titanium dioxide. Unfortunately, this pigment has one of thehighest carbon footprints of all the components in the paint. It istherefore important that it is used as efficiently as possible.

It is known that as the level of titanium dioxide in a paint increases,the efficiency per gram added decreases due to destructive interferencebetween closely adjacent titanium dioxide particles. In additionflocculation of the titanium dioxide during the drying process canexacerbate the problem. Thus controlling the spacing of the titaniumdioxide particles in a composition is critical to achieve maximumopacity per gram of the pigment added. This is especially important incoating compositions which start as liquids and dry to a solid film.Pigment particles can flocculate during the drying process.

In known coating compositions the spacing between titanium dioxide orother particles can be affected by a variety of factors including thepigment volume concentration (PVC), the choice of fillers and extenders.However, it is difficult to reliably control this spacing by simplyvarying these parameters. This is particularly so at high PVC.

US Patent Application Publication No. 2006/0009546 (Brown) discloses amethod for forming an opacifying particle comprising: providing,optionally in aqueous dispersion, a pigment particle of from 0.005 to 5μm average diameter and an index of refraction of at least 1.8;attaching a first polymer to the surface of said pigment particle via asilane residue; and, substantially encapsulating said pigment particlehaving said attached first polymer with a second polymer, wherein saidencapsulating is optionally effected by emulsion polymerizing at leastone ethylenically unsaturated monomer in the presence of said pigmentparticle having said attached first polymer.

European Patent Application Publication No. 2 161 304 A2 (Rohm & Haas)describes an opacifying pigment encapsulated in polymer which includes:a pigment particle having an average particle diameter of from 0.005 to5 microns and an index of refraction of at least 1.8; an aminophosphorusacid-functional first polymer having been used to disperse the pigmentparticle in an aqueous medium; and, a second polymer that at leastpartially encapsulates the dispersed pigment particle.

US Patent Application Publication No. 2006/0222851 (Miyabayashi et al.)describes encapsulation of a core material having a surface charge witha plurality of coating layers using a process comprising: (1) addingionic polymerizable surfactant A and/or ionic monomer, each having acharge opposite to the surface charge of the core material, to anaqueous dispersion of the core material having the charge on the surfacethereof, followed by mixing; then, (2) adding ionic polymerizablesurfactant B followed by mixing, emulsification and addition of apolymerization initiator thereto to conduct polymerization in water,thus forming the first coating layer; subsequently, (3) adding ionicpolymerizable surfactant C and/or an ionic monomer, each having a chargeopposite to the surface charge of the first coating layer to an aqueousdispersion of coated matter having the first coating layer, followed bymixing; and, thereafter, (4) adding ionic polymerizable surfactant D,followed by mixing, emulsification and addition of polymerizationinitiator to conduct polymerization in water, thus forming the secondcoating layer.

International application WO 2011/066608 discloses a macro controlledReversible Addition-Fragmentation chain Transfer (or RAFT) process usedto polymerise monomers to form a first polymer from the surface of apigment in water, where said first polymer uniformly encapsulates thepigment and stabilises the pigment particles to form stable aqueousdispersion in water. Onto the particles of the dispersion arepolymerised further monomers to produce a base swellable polymer andonto this are polymerised still more monomers to produce extensiblepolymer.

The first polymer, base swellable polymer and extendable polymer arepolymerised using a RAFT process and consequently a linear blockcopolymer is formed, attached to the pigment particle. The three regionsmentioned are covalently linked; the first polymer is covalently linkedto the base swellable polymer and the base swellable polymer iscovalently linked to the extensible polymer. Of course this differs fromsequential emulsion polymerisation, where the different emulsionpolymers are not covalently linked but rather are simply in physicalcontact at the boundaries.

Raising the pH of the swellable polymer under suitable conditions shouldcause the swellable polymer to swell and the extensible polymer also toswell (to accommodate the increased volume). On drying, the baseswellable polymer loses water and the pigment finds itself in an airvoid. Scattering and thus opacity is improved in such circumstancesbecause the amount destructive interference is reduced. Furthermore, thepresence of voids in the particles increases the overall refractiveindex difference between the binder medium and the pigment.

RAFT agents have the general formula (S)(Z)C(S[X]_(n)R¹)

where each X is independently a polymerised residue of an ethylenicallyunsaturated monomer, n is an integer from 0 to 100, R¹ and Z are groupsindependently selected such that the agent can function as a RAFT agentin the polymerisation of the selected one or more ethylenicallyunsaturated monomers.

Unfortunately, RAFT type polymerisation is a complex, expensive and slowprocess. In fact the process is so slow that typical cycle times, thetime taken from start to finish, can be 24 hours. This is unacceptablylong for an industrial process and it is economically unviable.

Prior art attempts using conventional free radical polymerisation, asdescribed in WO2011/066608, have proved unsuccessful, producingdefective particles including having non-uniform shell polymer.

What is required is a simple and efficient process for making suchparticles.

Surprisingly, we have now found that we can make hollow polymerparticles containing pigment particles therein using a non-RAFT emulsionpolymerisation process using free radical polymerisation. The techniqueis much faster and simpler than the RAFT prior art method previouslydisclosed.

Accordingly, there is provided an aqueous dispersion of pigmentedopacifying polymer particles, the particles comprising:

-   -   i. an inorganic core comprising dispersed inorganic pigment        particle and a dispersant    -   ii. a first layer of base-swellable polymer comprising acid        groups encapsulating the core    -   iii. a second layer of non-base-swellable polymer encapsulating        the first layer        wherein the dispersant comprises or consists of hydroxyl        moieties; and carboxyl groups derived from itaconic acid

Preferably, the aqueous dispersion of the invention comprises anintermediate polymer layer between the pigment and the base swellablepolymer layer and which is free of an intermediate polymer layer betweenthe base swellable polymer layer and the non-base swellable polymerlayer.

Preferably, the monomers of step v. used to form the intermediatepolymer comprise acid functional monomers and more preferably, the acidfunctional monomer is free of carboxyl moieties; even more preferablythe monomer comprises sodium styrene sulphonate. This ensures that thepigment dispersion is stable (that is, it does not flocculate or becomeshear unstable) at the low pH necessary for step ii. By low pH in thiscontext we mean less than pH 4 and more preferably between pH 2 and 4.Preferably, the intermediate polymer comprises from 5 to 30 wt % ofsodium styrene sulphonate.

Preferably, the amount of the intermediate polymer required to ensurethat the pigment dispersion is stable at the low pH is at least 7.5 wt %based on the weight of the pigment.

The presence of an intermediate polymer layer between the pigment andthe first layer of base swellable polymer normally ensures the necessarystability, at least to low pH. In such circumstances, a secondintermediate polymer layer between the base swellable polymer and thenon-base swellable polymer, is not required. However, forming such apolymer can be advantageous for other reasons.

Dispersants comprising itaconic acid and hydroxyl moieties are stable inthe conditions prevailing during the polymerisation stages of theprocess of preparing the particles. Furthermore, the long term storagestability of the dispersion of opacifying polymer particles is improvedover prior art dispersants. By stable we mean that the dispersant andthe pigment dispersion comprising it do not flocculate duringpolymerisation.

Optionally, intermediate polymer layers of acid value from 0 to 100 mgKOH/g non volatile (non vol) polymer are present between the core andthe first layer of base swellable polymer and/or between the first layerof base-swellable polymer and the second layer of non-base swellablepolymer.

Preferably, the polymer layers are free of RAFT agents.

Preferably, the polymer layers are free of agents containingthiocarbonylthio moieties.

Suitable inorganic pigment particles for use in the present inventioninclude barium sulphate, calcium carbonate, zinc sulfide, lithopone,metal oxides including zinc oxide, antimony oxide, zirconium oxide,chromium oxide, iron oxide, lead oxide, and forms of titanium dioxidesuch as anatase and rutile. Preferably, the inorganic pigment particlesare titanium dioxide particles, in particular particles selected fromrutile titanium dioxide and anatase titanium dioxide.

In another aspect of the invention there is provided a composition,preferably a coating composition, comprising the aqueous dispersion ofthe invention.

In another aspect of the invention there is provided a method ofpreparing the aqueous dispersion of opacifying polymer particles of theinvention, the particles containing inorganic pigment particles therein,comprising the steps of

-   -   i. dispersing inorganic pigment in aqueous medium using a        dispersant, of acid value at least 65 mg KOH/g of dispersant,        -   wherein the dispersant comprises or consists of carboxyl            acid moieties of differing pKa derived from itaconic acid,            and hydroxyl groups and        -   the pH during dispersion is greater than the pKa of the            carboxyl moiety having the lower pKa        -   and also at least 0.25 pH units above or below the            iso-electric point of the pigment, to form a stable aqueous            dispersion of the pigment in the aqueous medium.    -   ii. adjusting the pH of the pigment dispersion to below the pKa        of the acid moiety comprising the base swellable polymer of        step iii. whilst satisfying the pH criteria of step i.    -   iii. forming a first layer of a base swellable polymer on the        pigment particles by polymerising monomers comprising acid        groups    -   iv. forming a second layer of non-base swellable polymer    -   v. optionally polymerising further monomers to form intermediate        polymer layers having acid value between 0 and 65 mg KOH/g        polymer between the pigment and the first layer and/or between        the first and second layer    -   vi. at least partially neutralising the first polymer, using a        base, at a temperature above the effective Tg of the second        layer of non-swellable polymer whereby the first layer of        polymer is caused to swell; and,    -   vii. cooling the dispersion to a temperature below the effective        Tg of the second layer.

Itaconic acid (1-propene-2, 3 dicarboxylic acid) comprises two carboxylmoieties of different pKa, specifically at 3.85 and 5.45 respectively.The lower pKa acid moiety ionises more readily and it is the pKa of thiscarboxyl that is to be taken into account for the purposes of thepresent invention.

In another aspect of the invention the dispersion of opacifying polymerparticles is provided as a dry powder by drying the dispersion usingsuitable drying means. Suitable means include spray drying or freezedrying. In this form the particles may be used in compositions which arefree of volatile materials.

In another aspect of the invention, there is provided a coatingcomposition comprising the aqueous dispersion or the dried opacifyingpolymer particles of the invention.

Tg denotes the glass transition temperature of a polymer. The Tg of acopolymer can be calculated using the Fox equation where1/Tg=W₁/Tg₁+W₂/Tg₂+W₃/Tg₃+ . . . where Tg₁, Tg₂, Tg₃ . . . are the glasstransition temperatures of the homopolymers of the component monomers in^(o)K and W₁, W₂, W₃ . . . are the weight fractions present.

Polymers can be softened by the use of plasticisers. Such plasticiserseffectively reduce the Tg of the polymer. Coalescing solvents areplasticisers and encourage film formation of coatings comprising polymerparticles by softening at least the outer regions of the polymerparticles to the extent that the merge to form a continuous orsemi-continuous film. Some polymers may also be plasticised by monomers.The extent to which a plasticizer, coalescing solvent and/or monomerreduces the calculated Fox Tg of a particular polymer may be calculatedaccording to equation 1:1/Tg _(eff) =V _(p) /Tg _(p) +αV _(s) /Tg _(s)  Equation 1

Where: Tg_(p) and Tg_(s) is the glass transition temperature of thepolymer and the solvent respectively; Tg_(eff) is the effective glasstransition temperature of the polymer in the presence of theplasticizer; V_(p) and V_(s) are the volume fractions of the polymer andthe solvent respectively; a is a factor accounting for small changes inplasticising efficiency and is assumed to be 1 for the purposes of thisinvention.

Pigment Dispersant

The dispersant preferably, has a weight average molecular weight (Mw) ofat least 1000 Daltons, more preferably, from 1000 to 100000 Daltons,more preferably from 1500 to 50000 Daltons. The dispersant may be linearor non-linear polymers including comb, block and star types.

Preferably, the acid value of the dispersant is at least 100 mg KOH/gdispersant, more preferably from 100 to 450 mg KOH/g dispersant, evenmore preferably from 150 to 400 mg KOH/g dispersant.

Preferably, the hydroxyl value of the dispersant is from 50 to 400 mgKOH/g dispersant, more preferably 100 to 250 mg KOH/g dispersant. Thehydroxyl group may be introduced into the dispersant throughethylenically unsaturated monomers containing hydroxyl groups. Suitablesuch monomers include hydroxy ethyl acrylate, hydroxy ethyl methacrylateand hydroxy isopropyl methacrylate.

In addition it is preferable that the pigment dispersant containsadditional moieties that adsorb to the surface of the pigment or thesurface treatment on the pigment. In the case of titanium dioxide usedin surface coatings, this is often surface treated with alumina, silicaand zirconia. Moieties that adsorb to these are desirable. Suitablemoieties include amines—including heterocyclics such as morpholine.

The dispersant may be made using any polymerisation method includingfree radical addition polymerisation, condensation polymerisation, andcontrolled radical polymerisation (CRP) including RAFT polymerisation,macromolecular design by interchange of xanthate polymerisation (MADIX),nitroxide mediated polymerisation (NMP) and atom transfer radicalpolymerisation (ATRP).

The dispersant is preferably made in a good solvent for the dispersantso that a solution of dispersant in solvent is formed. Suitable solventsinclude water compatible organic solvents, aqueous mixtures containingless than 50 wt % of organic solvent; or water. More preferably, aqueousmixtures are used.

Preferably, the pigment dispersants are used at from 0.1 to 4, morepreferably from 1 to 2 wt % based on the weight of titanium dioxide. Theamount of pigment dispersant required will vary, mainly in accordancewith the particle size and the density of the particular pigment.

Architecture of the Opacifying Polymer Particles

The dispersion of opacifying polymer particles of the invention comprisea particle of titanium dioxide encapsulated by a layer of base swellablepolymer, the base swellable polymer encapsulated by anon-base-swellable, non-film-forming polymer.

Optionally, a third layer of film forming polymer is formed onto thesecond layer of non-base swellable, non-film forming polymer.

This has the unique advantage of providing the pigment in a form thatalso film forms.

The dispersed inorganic pigment particles suitable for use in theinvention preferably have a mean particle diameter of from 75 to 300 nm,more preferably from 100 to 300 nm, even more preferably from 150 to 300nm and most preferably from 200 to 300 nm.

When the inorganic pigment particles used are titanium dioxide, the meanparticle size of the titanium dioxide particles are preferably from 75to 150 nm in diameter.

The amount of base swellable polymer should increase the radius of theopacifying polymer particles by about from 2 to 20 nm before swellingand the layer of non-base-swellable, non-film-forming polymer, shouldincrease the radius by about a further 15 to 150 nm, preferably 30 to 75nm before swelling.

On adding base to the aqueous dispersion the base swellable layer swellswith water. Sufficient base should be added to swell the base swellablelayer so that the radius increases to from 10 to 200 nm.

Of course, in order for the base-swellable polymer layer to swell, thenon-base-swellable polymer must itself be extensible during the swellingstep.

On drying, the dispersion of the invention naturally loses water. Thewater swelling the base-swellable polymer migrates through the particleand escapes to the atmosphere leaving an air-filled void in which sitsthe particle of titanium dioxide.

FIG. 1 shows an idealised structure of an opacifying particle comprisingthe invention where (1) is a pigment particle, (2) is a layer ofbase-swellable polymer in its unswollen state and (3) is the layer ofnon-base-swellable polymer.

FIG. 2 shows the same structure as in FIG. 1 at basic pH with thebase-swellable polymer (2) swollen with water.

FIG. 3 shows an idealised structure of an opacifying particle comprisingthe invention where (1) is a pigment particle, (2) is a layer ofbase-swellable polymer in its unswollen state, (3) is the layer ofnon-base-swellable polymer and (4) and (S) are intermediate layers.

BASE SWELLABLE POLYMER

The role of the base swellable layer is to react with a base once it hasbeen at least partially overcoated with a rigid outer shell to form ahydrophilic ionomeric layer. This layer causes surrounding water to bedrawn in by osmosis and to swell the polymer.

The base swellable polymer is preferably derived from one or moremonomers selected from the group consisting of methacrylic acid, acrylicacid crotonic acid, maleic acid or maleic anhydride, itaconic acid,cinnamic acid, fumaric acid and beta carboxy ethyl acrylate.

This base swellable polymer layer should preferably contain sufficientacid monomer to give an acid value of from 100 to 450 mg KOH/g ofpolymer more preferably 130 to 260 mg KOH/g of polymer. This correspondsto from about 15 to 70 wt % and 20 to 40 wt % of methacrylic acidcalculated on the total weight of base-swellable polymer.

The base-swellable-polymer layer can also contain non-polymerisable acidif desired. Suitable non-polymerisable acid monomers include C₆-C₁₂aliphatic moncarboxylic acids and aromatic acids such as benzoic acid.

The base-swellable-polymer layer may optionally be crosslinked with amultifunctional unsaturated monomer such as divinyl benzene, diallylmethacrylate, ethylene glycol dimethacrylate, butane diol dimethacrylateor allyl methacrylate. Preferably, from 0.1 to 5 wt. %, based on theweight of base swellable polymer, of said multifunctional unsaturatedmonomer is used, more preferably from 0.1 to 1.0 wt % is used. Allylmethacrylate is particularly preferred as a crosslinker as it promotesgrafting between the different polymer layers.

The Fox Tg of the base-swellable-polymer layer is preferably from 0 to170° C., and more preferably from 20 to 150° C.

The thickness of the base-swellable-layer is from 2 to 20 nm unswollen,and from 10 to 200 nm swollen. By thickness is meant as measured by theincrease in radius of the particle.

Non-Base-Swellable Polymer

The non-base swellable polymer must fulfil two roles. Firstly, duringthe neutralisation step of the base-swellable polymer it must expand,without rupturing, in order to accommodate the increased volume—thus, itmust be chosen to have an effective Tg below the temperature used duringthe neutralisation step. Secondly, during drying and loss of water, thenon-base-swellable polymer must be rigid enough to prevent collapse ofthe void. The polymer should preferably have a high modulus.

Preferably the non-base-swellable polymer is non-film forming at ambienttemperature. Non-film forming polymers have a Tg_(eff) above ambienttemperature.

A good guide to modulus is the glass transition temperature, Tg.Generally, as Tg increases, modulus also increases and we have foundthis can be used as a useful guide to determine the Tg of the polymerwhich resists collapse and film formation.

Preferably, the Fox Tg of the non-base-swellable polymer is at least 60°C., more preferably from 90 to 140° C.

Furthermore, the polymer is preferably not significantly plasticised bywater as this will cause it to soften and deform, especially as thepaint dries. Preferably it is hydrophobic as this reduces waterplasticisation and it has the added benefit that it improves waterresistance of coatings containing the opacifying polymer particles ofthe present invention. More preferably the solubility of the monomersmaking up the non-base-swellable polymer should be less than 2 g/100 gwater at 20° C.

Suitable monomers to make the non-base-swellable polymer includestyrene, alpha methyl styrene (as a monomer in a copolymer), tert-butylstyrene, vinyl toluene, methyl methacrylate, acrylonitrile,methacrylonitrile and copolymers of these with lower Tg monomers.

Preferably the polymer contains 90-100% styrene monomer. It may alsocontain from 0 to 20% acidic moieties, more preferably from 0 to 10%.These are especially beneficial because it helps the non-base-swellablelayer adhere to the base-swellable polymer layer. The presence of theacid moieties also helps with the swelling stage by helping tofacilitate passage of the neutralising base through the rigid outershell during the neutralisation step.

The acid value of the non-base-swellable polymer layer is preferablyless than 130, more preferably less than 100 even more preferably lessthan 65 mg KOH/g non vol. polymer.

The thickness (i.e. the radius) of the non-base-swellable polymer layeris preferably from 15 to 150 nm, more preferably from 30 to 75 nm. Ofcourse the thickness of the non-base-swellable polymer reduces as thebase-swellable polymer swells because it has a greater area toencapsulate.

Optional Intermediate Polymer Layer

The optional intermediate polymer layer is not base-swellable.

Preferably, the acid value of the optional intermediate polymer layer isfrom 0.1 to 65.0 mg KOH/g of polymer, more preferably from 10 to 65 mgKOH/g of polymer. The presence of some acid functional monomers isthought to help the passage of base, especially inorganic base such asNaOH, through the intermediate layer. Suitable acid functional monomersare as hereinbefore described in relation to the base swellable polymerlayer. Additionally, monomers comprising strong acid moieties such assulphonate and phosphate can be used. Suitable examples include sodiumstyrene sulphonate.

The Fox Tg of the intermediate polymer layer is preferably between 30and 100° C. and more preferably between 50 and 90° C.

The thickness of the optional intermediate polymer layer is 5 to 50 nmbefore swelling of the base swellable polymer layer, more preferably 10to 30 nm.

Without being bound by this, it is thought that the optionalintermediate polymer layer at least partially or preferably fullyencapsulates the base swellable polymer layer in order to make it easierfor the far less polar rigid outer shell to overcoat the base swellablepolymer in a more uniform manner.

Swelling Stage

The base swellable polymer layer is swollen by raising the pH using, forexample, either volatile bases, including ammonia or amine, ornon-volatile bases, for example alkali metal hydroxides such as sodiumhydroxide. Of course, the polymer is not actually swollen by base butrather by the aqueous medium or water comprising the continuous phase ofthe dispersion. Following addition of the base, the neutralised acidmoieties on the polymer become hydrophilic and the aqueous phase orwater is drawn in by osmosis creating the driving force for swelling.

In order for the water ingress to proceed unhindered, thenon-base-swellable non-film-forming polymer must be extensible duringthe neutralisation stage. It is necessary that the addition of the baseis carried out at a temperature above the effective glass transitiontemperature of the non-base-swellable non-film-forming polymer in orderto accommodate the increase in volumes accompanying the water ingresswithout rupturing.

An aqueous medium comprises at least 50 wt % of water, the remaindercomprising organic solvents, preferably water compatible solvents.

In the absence of a plasticising material for the polymer, the effectiveTg is the same as the Fox Tg. Where a plasticiser is present theeffective Fox Tg is lower than the Fox Tg.

Preferably, the addition of base is at a temperature at least 5° C.,more preferably from 5 to 20° C. above the effective Tg of the polymer.

For example where the base swellable layer is overcoated with anintermediate polymer layer of Tg less than the process temperature, thebase can be added to the dispersion at elevated temperature (>Tg of theintermediate layer) and after the swelling is complete and the pH hasdropped, a rigid hydrophobic layer of polymer is polymerised on top ofthis. Alternatively, the addition and polymerisation of the monomer canoccur whilst the swelling is occurring.

Alternatively, a rigid hydrophobic layer of polymer can be polymeriseddirectly onto the base swellable polymer layer, followed by plasticisingthis polymer layer with monomer (in order to reduce its effective Tg).This can be achieved by (a) stopping the addition of initiator, (b)optionally adding a free radical inhibitor (e.g. monomethyl etherhydroquinone, MEHQ), (c) using a non-homopolymerisable monomer, or (d)using a monomer with a ceiling temperature below the operatingtemperature (e.g. alpha methyl styrene). The base is then added to thedispersion at elevated temperature as before and after the swelling iscomplete and the pH has dropped, the polymerisation is continued (e.g.by adding further initiator and monomer) and a rigid hydrophobic layerof polymer is polymerised on top of this. Of course, a plasticisingsolvent can be also be used but is far less preferable as, since,depending on its boiling point, it may well contribute to VOC.

Polymerisation Method

The polymerisation steps of the present invention are carried out usinga sequential emulsion polymerisation process in the presence ofinorganic pigment particles, preferably titanium dioxide, dispersed inaqueous medium, preferably water.

By sequential is meant that monomer mixtures of different compositionare polymerised one after the other. In the simplest example of such amethod, particles may be made having a first polymer or core regiondiffering in composition from a second or shell polymer region. Ofcourse, the particles may have more than two polymer regions.

Preferably, the polymers are made using unconstrained free radicalpolymerisation methods, more preferably using free radical emulsionpolymerisation methods. Even more preferably free radical initiators areused.

The monomers are preferably emulsified in water and surfactant and fedinto the reactor vessel over a period of from 1 to 6 hours, preferablyfrom 1 to 3 hours. Consecutively, the free radical initiator is fed intothe reactor.

It is advantageous to have a delay between the various monomer feeds. Inthe non-RAFT type polymerisation used in this invention, the growingchains are very short-lived. The delay, therefore, ensures that anygrowing chains of the previous polymerisation terminate and stop growingbefore the next monomer mixture is polymerised.

Preferably the titanium dioxide pigment particles are dispersed in waterin the form of primary particles (i.e. with the minimum level ofagglomerates). More preferably, the dispersed pigment has a meanparticle diameter of from 150 to 300 nm, even more preferably from 200to 300 nm and most preferably from 225 to 275 nm as measured by dynamiclight scattering.

The polymerisation is carried out in the absence of RAFT chain transferagents, preferably in the absence of all types of controlled radicalpolymerisation.

Processing Conditions

It is preferable to run the polymerisation under conditions thatencourage control of the morphology, e.g. low free monomer levels andlevels of surfactant that are adequate for giving stability but not sohigh as to cause nucleation and stabilisation of non pigmented latexparticles. As a corollary to this, it may be useful to make adjustmentsto lower the pH of the dispersion of inorganic pigment particles formedafter step i) of the process—for instance, by the addition thereto of anacid such as hydrochloric acid—and allow for pH equilibration before thepolymerization of the base swellable polymer.

The invention will now be illustrated by the following examples, inwhich the abbreviations used are defined below.

The abbreviations below have the following meanings

AMA Allyl Methacrylate

AMPS 2-Acrylamido-2-Methylpropane Sulfonic Acid DELETE ???

AMS Alpha-Methyl Styrene DELETE ???

BA Butyl Acrylate

BMA Butyl Methacrylate

DI water Deionised water

DMAEMA Dimethylaminoethyl Methacrylate DELETE ???

DVB Di-Vinyl Benzene DELETE ???

HEMA Hydroxyethylmethacrylate

IA Itaconic Acid

MAA Methacrylic Acid

MAM Methacrylamide

MMA Methyl Methacrylate

SSS Styrenesulfonic Acid Sodium salt hydrate

ST Styrene

Example 1 Anionic Dispersant Solution (D−1)

A solution of anionic polymer dispersant was produced according to theprocess described below and in Table 1, and having a monomer compositionMMA:HEMA:IA:MAM of 15:45:36:4 by weight.

TABLE 1 Dispersant Polymer (P1) Material Weight (g) Solvent (A)Iso-Propanol 15.17 DI water 5.06 Monomer (B) Methyl Methacrylate 7.05Hydroxyethylmethacrylate 20.97 Itaconic Acid 16.68 Methacrylamide 1.91n-Octyl Mercaptan 1.05 VAZO 67 1.46 Iso-Propanol 22.58 DI Water 7.75Initiator (C) VAZO 67 0.11 Iso-Propanol 0.24 DI Water 0.14 Total 100.17

The solvent mixture (A) was loaded to a reaction vessel. 25% of themixture (B) was then added and the temperature increased to 65° C. undera blanket of nitrogen.

The mixture was then allowed to exotherm to 82° C. and held for 15 min.The remainder of the mixture (B) was then fed over 2 h at 85° C. Themixture (C) was then added and the mixture held at reflux temperature(80 to 85° C.) for 2 h.

A Dean Stark was then adapted to the reaction vessel and theIso-Propanol was distilled off. As described in Table 2, (D) and (E)were then added while stirring at high speed at 75° C. to form thepolymer solution (D−1) at 27.3 wt % solids.

TABLE 2 Anionic Dispersant Solution D-1 Material Weight (g) Polymer (P1)33.50 DI Water (D) 62.50 Ammonia (E) 4.00 Total 100.00

Aqueous dispersion of titanium dioxide MB-1

The anionic dispersant solution, D−1, was used to make an aqueousdispersion of titanium dioxide according to the recipe and method below.

TABLE 3 Material Weight (g) D-1 23.01 DI Water 106.24 Tipure R-706420.75 Total 550.00

23.01 g of the dispersant solution (D−1) were diluted in 106.24 g ofwater. 420.75 g of Tipure R-706 were then dispersed in the solutionobtained using a high speed disperser operating at 1,500 rpm for 20 min.

Aqueous Dispersion of Opacifying Polymer Particles

An aqueous dispersion of opacifying polymer particles was made using thetitanium dioxide dispersion prepared in Table 3. The method andingredients used are shown in Table 4.1 and 4.2

TABLE 4.1 TiO2 dispersion with first polymer layer (ME-1) MaterialWeight (g) Aqueous charge (A) MB-1 261.90 Sodium Dodecyl BenzeneSulphonate 1.46 Solids adjust (B) DI water 258.68 Initiator Catalyst (C)0.1% Iron II Sulphate aqueous solution 1.12 1% Ethylene Diamine TetraAcetic Acid aqueous solution 0.08 Initiator Feed (D) Tert-ButylHydroperoxide (70% active) 0.37 DI water 3.27 Reductant Feed (E)Ascorbic Acid 0.20 DI water 15.04 Optional non-base swellable polymerlayer (F) DI water 5.38 Sodium Dodecyl Benzene Sulphonate 0.64 ButylMethacrylate 10.93 Methyl Methacrylate 6.25 Styrenesulfonic Acid Sodiumsalt hydrate 1.93 Total 567.69

The TiO₂ dispersion (A) was charged to a reaction vessel, diluted withDI (B), purged with nitrogen and the temperature raised to 50° C. Thecatalyst (C) was then added followed two minutes later by the initiator(D). The reductant (E) and the monomer mixture (F) were then fed inlinearly over 30 min at 50° C.

TABLE 4.2 Material Weight (g) Aqueous charge ME-1 567.69 Solids Adjust(G) DI Water 154.89 Initiator Catalyst (H) 0.1% Iron II Sulphate aqueoussolution 1.48 1% Ethylene Diamine Tetra Acetic Acid aqueous solution0.17 Sodium Dodecyl Benzene Sulphonate 4.93 Initiator Feed (I)Tert-Butyl Hydroperoxide 1.19 DI Water 11.18 Reductant Feed (J) SodiumFormaldehyde Sulfoxylate 0.42 DI Water 11.13 Base swellable layer stage(K) DI Water 7.08 Sodium Dodecyl Benzene Sulphonate 3.26 MethylMethacrylate 16.01 Butyl Acrylate 16.01 Methacrylic Acid 16.01 AllylMethacrylate 0.05 Solids Adjust (L) DI Water 234.84 Initiator Catalyst(M) 0.1% Iron II Sulphate aqueous solution 1.13 1% Ethylene DiamineTetra Acetic Acid aqueous solution 0.13 Initiator Feed (N) Tert-ButylHydroperoxide 0.92 DI Water 8.63 Reductant Feed (O) Sodium FormaldehydeSulfoxylate 0.32 DI Water 8.46 Optional Intermediate Polymer layer (P)Methyl Methacrylate 33.10 Butyl Acrylate 3.68 Aerosol MA80 0.87 DI Water8.83 Non film forming outer shell (Q) Methyl Methacrylate 55.21 AerosolMA80 1.31 DI Water 13.24 Initiator/Feed (R) Ammonium Persulphate 0.34 DIWater 19.51 Ammonia addition (S) Ammonia 45.00 Total 1247.00

ME-1 and Solids Adjust (G) were charged to a reaction vessel. Thedispersion was then adjusted to pH 4.1 by addition of hydrochloric acid(1 mol/L) and stirred for 10 min to allow for pH equilibration. Afteraddition of (I), the pH was checked again and readjusted if necessary to4.1 by addition of hydrochloric acid (1 mol/L). The solution was purgedwith N₂ and the temperature adjusted to 50° C. (J) and (K) were fed inlinearly over 30 min, before the solid adjust (L) was added.

The temperature was then kept at 50° C. while adding (M) and (N); (0)and (P) were then fed over 30 min.

The dispersion was then heated up to 90° C. and (Q) and (R) were fedinto the vessel.

The mixture was then allowed to cool to room temperature; it wasfiltered to remove any grit. The filtered dispersion was then stirred at90° C. and ammonia (S) added over 30 min. The stirring was continued for4 hours at 90° C., before the dispersion was allowed to cool and wasfiltered.

Example 2

As for Example 1, with the following formulation changes:

Base swellable layer stage (K) DI Water 7.13 Sodium Dodecyl BenzeneSulphonate 3.29 Methyl Methacrylate 20.78 Butyl Acrylate 13.07Methacrylic Acid 14.53 Allyl Methacrylate 0.05 Optional IntermediatePolymer layer (P) Styrene 36.78 Aerosol MA80 0.87 DI Water 8.83 Non filmforming outer shell (Q) Styrene 55.21

All other process and formulation steps remaining identical.

Example 3

As for Example 1, with the following formulation changes:

Base swellable layer stage (K) DI Water 7.02 Sodium Dodecyl BenzeneSulphonate 3.24 Methyl Methacrylate 11.84 Butyl Acrylate 16.75Methacrylic Acid 19.06 Allyl Methacrylate 0.05 Non film forming outershell (Q) Styrene 54.79 Aerosol MA80 1.30 DI Water 13.14

All other process and formulation steps remaining identical.

Example 4

As example 2, with the following formulation changes:

Base swellable layer stage (K) DI Water 7.08 Sodium Dodecyl BenzeneSulphonate 3.26 Methyl Methacrylate 16.01 Butyl Acrylate 16.01Methacrylic Acid 16.01 Allyl Methacrylate 0.05

All other process and formulation steps remaining identical.

Example 5

As for example 1, with the following formulation changes:

Non film forming outer shell (Q) Styrene 56.18 Aerosol MA80 1.31 DIWater 13.24

All other process and formulation steps remaining identical.

Example 6

As for example 1, with the following formulation changes:

Optional Intermediate Polymer layer (P) Methyl Methacrylate 31.72 ButylAcrylate 5.06 Aerosol MA80 0.88 DI Water 8.83 Non film forming outershell (Q) Styrene 53.44 Aerosol MA80 1.36 DI Water 12.81

All other process and formulation steps remaining identical.

Example 7

As for example 1, but with the following changes:

Material Weight (g) Aqueous charge (A) MB-1 262.81 Sodium DodecylBenzene Sulphonate 1.47 Solids adjust (B) DI water 259.58 InitiatorCatalyst (C) 0.1% Iron II Sulphate aqueous solution 1.12 1% EthyleneDiamine Tetra Acetic Acid aqueous solution 0.08 Initiator Feed (D)Tert-Butyl Hydroperoxide (70% active) 0.37 DI water 3.28 Reductant Feed(E) Sodium Formaldehyde Sulfoxylate 0.20 DI water 15.09 Optional nonbase swellable polymer layer (F) DI water 5.40 Sodium Dodecyl BenzeneSulphonate 0.65 Butyl Methacrylate 11.28 Methacrylic Acid 0.39 MethylMethacrylate 5.98 Styrenesulfonic Acid Sodium salt hydrate 1.96 SolidsAdjust (G) DI Water 155.43 Initiator Catalyst (H) 0.1% Iron II Sulphateaqueous solution 1.48 1% Ethylene Diamine Tetra Acetic Acid aqueoussolution 0.17 Sodium Dodecyl Benzene Sulphonate 4.95 Initiator Feed (I)Tert-Butyl Hydroperoxide 1.19 DI Water 11.22 Reductant Feed (J) SodiumFormaldehyde Sulfoxylate 0.42 DI Water 11.17 Base swellable layer stage(K) DI Water 7.10 Sodium Dodecyl Benzene Sulphonate 3.27 MethylMethacrylate 20.70 Butyl Acrylate 13.02 Methacrylic Acid 14.47 AllylMethacrylate 0.05 Solids Adjust (L) DI Water 235.66 Initiator Catalyst(M) 0.1% Iron II Sulphate aqueous solution 1.14 1% Ethylene DiamineTetra Acetic Acid aqueous solution 0.13 Initiator Feed (N) Tert-ButylHydroperoxide 0.92 DI Water 8.66 Reductant Feed (O) Sodium FormaldehydeSulfoxylate 0.32 DI Water 8.49 Optional intermediate Polymer layer (P)Methyl Methacylate 33.21 Butyl Acrylate 3.69 Aerosol MA80 0.87 DI water8.86 Non film forming outer shell (Q) Styrene 55.40 Aerosol MA80 1.31 DIwater 13.29 Initiator/Feed (R) Ammonium Persulphate 0.34 DI Water 19.58Ammonia (S) Ammonia 40.81 Total 1246.98

All process steps remain unchanged.

Example 8

Same as example 1, but with the following formulation changes:

Optional non base swellable polymer layer (F) DI water 5.42 SodiumDodecyl Benzene Sulphonate 0.65 Butyl Acrylate 4.18 Methyl Methacrylate13.54 Styrenesulfonic Acid Sodium salt hydrate 1.97 Base swellable layerstage (K) DI Water 7.13 Sodium Dodecyl Benzene Sulphonate 3.29 MethylMethacrylate 20.78 Butyl Acrylate 13.07 Methacrylic Acid 14.53 AllylMethacrylate 0.05 Non film forming outer shell (Q) Styrene 55.61 AerosolMA80 1.32 DI Water 13.34 Ammonia addition (S) Ammonia 36.17

All other formulation and process steps remaining unchanged.

Example 9

Same as example 5, with the following formulation changes.

Optional non base swellable polymer layer (F) DI water 5.38 SodiumDodecyl Benzene Sulphonate 0.64 Styrene 0.39 Butyl Methacrylate 10.93Methyl Methacrylate 6.69 Styrenesulfonic Acid Sodium salt hydrate 1.93

All other formulation and process steps remaining unchanged.

Spreading Rates

The spreading rate to achieve a contrast ratio of 95% and 98% wasevaluated by converting examples 1-3, 5-7 and 9 of the dispersion tomodel paints according to the formulations shown in Table A

TABLE A Example 1 2 3 5 6 7 9 wt (g) wt (g) wt (g) wt (g) wt (g) wt (g)wt (g) Opacifier dispersion 46.72 50.01 49.19 32.85 47.26 44.66 48.90 ofexample indicated above Film forming latex 35.50 34.30 33.03 33.57 32.8333.94 33.64 VA:BA Thickener 1.50 1.50 1.49 1.48 1.51 1.51 1.50 Water15.98 13.89 15.99 31.79 18.10 19.59 15.66 AMP 95 0.30 0.30 0.30 0.300.30 0.30 0.30 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00PVC 29.57 31.95 34.43 33.35 34.80 32.66 33.27 TiO₂ PVC 8.09 8.72 8.748.64 8.84 8.53 8.62 Density 1.11 1.11 1.11 1.11 1.11 1.11 1.11 Volumesolids 25.00 25.00 25.00 25.00 25.00 25.00 25.00 (vol %) Weight solids32.54 32.52 32.49 32.51 32.48 32.47 32.47 (wt %)

Table B shows the recipes of two standard paints at PVC 9 and 10%.

TABLE B Standard A Standard B Standard TiO₂ slurry A Standard TiO₂slurry B Weight (g) Weight (g) — — TiO2 slurry (76.5 wt % 9.30 10.30TiO2) ASP 170 slurry (65 wt % 21.72 20.83 clay) VA-BA latex 30.18 30.08Thickener 1.50 1.49 DI 37.00 36.99 AMP 95 0.30 0.30 100.00 100.00 34.8034.82 TiO2 PVC % 9.00 10.00 Density 1.20 1.20 Volume solids % 25.0025.00 Weight solids 37.51 37.70

The TiO₂ and clay slurry recipes as used in Standard A and B are shownbelow

TiO₂ slurry ASP 170 slurry wt % wt % Water 22.80 34.30 Biocide (25%solids) 0.11 Dispersant (45% solids) 0.30 0.35 (Polyacrylic acid)Antifoam 0.20 AMP 95 0.04 Biocide (22% solids) 0.23 Antifoam (60%solids) 0.19 TiO₂ 76.48 Hydrous Clay 65.00 Total 100.00 100.00

The spreading rates are shown in Table C

TABLE C Spreading Rate Spreading Rate (m2/L at (m2/L at 95% 98% contrastRun Sample TiO₂ PVC PVC contrast ratio) ratio) 1 Example 1 8.09 29.577.3 4.5 2 Example 2 8.72 31.95 7.5 4.5 3 Example 3 8.74 34.43 7.3 4.2 4Example 5 8.64 33.35 7.3 4.4 5 Example 6 8.84 34.80 7.3 4.3 6 Example 78.53 32.66 7.2 4.2 7 Example 9 8.62 33.27 7.3 4.3 12 Standard A 9.0034.80 6.3 3.8 13 Standard B 10.00 34.82 7.0 4.4

As can be seen, at 95% contrast ratio all of the examples containingopacifying particle of the invention have better opacity, as evidencedby the increased spreading rates, compared to standard A and B at PVC of9 and 10%, even though the inventive compositions are at PVCsignificantly lower than these. Even at 98% contrast ratio all examplesof the invention have a better spreading rate than the standard A at PVCof 9%.

Effect of Intermediate Polymer on pH Stability of a Pigment Dispersion

Onto the pigment dispersion, MB-1 (as described in Table 3) waspolymerised various intermediate polymer comprising sodium styrenesulphonate.

The resulting dispersions were allowed to cool and hydrochloric acid wasadded, whilst stirring, to reduce the pH. The minimum pH reached beforeflocculation was noted.

Replacing sodium styrene sulphonate monomer with methacrylic acidmonomer in the intermediate polymer resulted in flocculation on additionof hydrochloric acid.

¹Intermediate ²SSS content of polymer/ intermediate Minimum pH reachedExample wt % polymer/wt % before flocculation 1 9.6 10.0 3.78 2 13.015.0 3.53 3 9.6 20.0 1.40 4 6.0 20.0 3.05 5 6.0 15.0 Flocculated at pH4.3 6 9.6 10.0 3.05 ¹wt % based on pigment ²SSS = sodium styrenesulphonate

The invention claimed is:
 1. An aqueous dispersion of pigmentedopacifying polymer particles, the particles comprising: i) an inorganiccore comprising a dispersed inorganic pigment particle and a dispersant,ii) a first layer of a base-swellable polymer comprising acid groupsencapsulating the core, and iii) a second layer of a non-base-swellablepolymer encapsulating the first layer; wherein the dispersant is apolymer having a weight average molecular weight (Mw) of at least 1000Daltons, and comprises hydroxyl moieties, and carboxyl groups derivedfrom itaconic acid.
 2. The aqueous dispersion according to claim 1,further comprising an intermediate polymer layer at least one of betweenthe pigment particle and the base-swellable polymer or between thebase-swellable polymer and the non-base swellable polymer.
 3. Theaqueous dispersion according to claim 2 comprising an intermediatepolymer layer between the pigment particle and the base swellablepolymer layer and which is free of an intermediate polymer layer betweenthe base swellable polymer layer and the non-base swellable polymerlayer.
 4. The aqueous dispersion according to claim 2 wherein theintermediate polymer layer has an acid value of from 0.1 to 65 mg KOH/gof polymer.
 5. The aqueous dispersion according to claim 1 wherein thepigment particle is a titanium dioxide particle.
 6. The aqueousdispersion according to claim 1, wherein the dispersed inorganicparticles have a mean particle diameter of from 75 to 300 nm.
 7. Theaqueous dispersion according to claim 6, wherein the acid value of thedispersant is from 65 to 400 mg KOH/g non-volatile (non-vol.)dispersant.
 8. The aqueous dispersion according to claim 1, wherein thebase swellable polymer is derived from one or more monomers selectedfrom the group consisting of methacrylic acid, acrylic acid, crotonicacid, maleic acid, maleic anhydride, itaconic acid, cinnamic acid,fumaric acid, and beta carboxy ethyl acrylate.
 9. The aqueous dispersionaccording to claim 1, wherein the non-base swellable polymer is non-filmforming at ambient temperature.
 10. The aqueous dispersion according toclaim 1, wherein the Fox Tg of the non-base swellable polymer is atleast 60° C.
 11. The aqueous dispersion according to claim 1, whereinsaid non-base swellable polymer comprises monomers selected from thegroup consisting of styrene, alpha methyl styrene, tert-butyl styrene,vinyl toluene, methyl methacrylate, acrylonitrile, methacrylonitrile,and mixtures thereof.
 12. The aqueous dispersion according to claim 11,wherein the non-base-swellable polymer contains 90-100% styrene monomer.13. A coating composition comprising the aqueous dispersion according toclaim
 1. 14. A substrate coated with the coating composition accordingto claim
 13. 15. A method of preparing the aqueous dispersion accordingto claim 1, comprising: i) dispersing an inorganic pigment in an aqueousmedium using a dispersant of acid value at least 65 mg KOH/g ofdispersant, wherein the dispersant is a polymer having a weight averagemolecular weight (Mw) of at least 1000 Daltons and comprises carboxylacid moieties of differing pKa derived from itaconic acid, and hydroxylgroups, wherein the pH during dispersion is greater than the pKa of thecarboxyl moiety having the lower pKa and at least 0.25 pH units above orbelow the iso-electric point of the pigment, to form a stable aqueousdispersion of the pigment in the aqueous medium; ii) adjusting the pH ofthe pigment dispersion to below the pKa of the acid moiety comprisingthe base swellable polymer of iii while satisfying the pH criteria of i;iii) forming a first layer of a base swellable polymer on the pigmentparticles by polymerising monomers comprising acid groups; iv) forming asecond layer of a non-base swellable polymer; v) optionally polymerisingfurther monomers to form intermediate polymer layers having an acidvalue between 0 and 65 mg KOH/g polymer between at least one of thepigment and the first layer or the first and second layer; vi) at leastpartially neutralising the first polymer, using a base, at a temperatureabove the Tg_(eff) of the second layer of non-swellable polymer wherebythe first layer of polymer is caused to swell; and, vii) cooling thedispersion to a temperature below the Tg_(eff) of the second layer. 16.The aqueous dispersion according to claim 4, wherein the intermediatepolymer layer comprises sodium styrene sulphonate.
 17. The aqueousdispersion according to claim 1, wherein the dispersed inorganicparticles have a mean particle diameter of from 225 to 275 nm.
 18. Theaqueous dispersion according to claim 1, wherein the Fox Tg of thenon-base swellable polymer is from 90 to 140° C.
 19. The aqueousdispersion according to claim 7, wherein the base swellable polymer isderived from one or more monomers selected from the group consisting ofmethacrylic acid, acrylic acid, crotonic acid, maleic acid, maleicanhydride, itaconic acid, cinnamic acid, fumaric acid, and beta carboxyethyl acrylate.
 20. The aqueous dispersion according to claim 8, whereinsaid non-base swellable polymer comprises monomers selected from thegroup consisting of styrene, alpha methyl styrene, tert-butyl styrene,vinyl toluene, methyl methacrylate, acrylonitrile, methacrylonitrile,and mixtures thereof.